A polyethylene resin such as low density polyethylene and the like has been generally crosslinked to enhance its mechanical strength, heat resistance and other properties. One such method of crosslinking ethylene polymers involves incorporating alkoxysilane functionality into the polymer structure either by grafting unsaturated alkoxysilanes onto the ethylene polymers or by direct copolymerization of ethylene with unsaturated alkoxysilanes, herein referred to as EVS copolymers.
The silane copolymer can be crosslinked by exposing the copolymer to moisture i.e., the silane is such that it makes the copolymer hydrolysable. The crosslinking is accelerated in the presence of an appropriate silanol condensation catalyst such as dibutyl tin dilaurate, dioctyl tin maleate, stannous acetate zinc octate stannous octate iron 2-ethyl hexoate and other metal carboxylates.
When polyethylene copolymerized with the unsaturated silane compound is used as electric cable insulation, there has been a problem in that the retention of dielectric strength after a mechanical glancing impact is not always satisfactory and especially unsatisfactory when flame retardants are incorporated into the compositions. The reasons for these problems are not fully understood.
Known typical flame retardant resin compositions used for the insulation of wire and cable, comprise antimony trioxide and a chlorine flame retardant or bromine flame retardant with a polyethylene. These halogen containing compositions such as, for example, bromine containing decabromodiphenyl oxide (DBDPO), works by releasing heavy non-flammable hydrogen halide (bromide) gases. Due to its high halogen content DBDPO (83.3% bromine), lower amounts of filler are required than with the chlorine containing 1,2,3,4,7,8,9,10,13,13,14,14-dodecachloro 1,4,4a,5,6,6a,7,10,10a,11,12,12a-dodecahydro 1,4,7,10-dimethanodibenzo (a,e) cyclooctene (Dechlorane Plus 25 (65.1% chlorine)). The latter is known to act as a better char promoter than the former.
However, there remains a need for flame retardant electric cable insulation which has both acceptable flame retardancy and dielectric strength after glancing impact.
In accordance with the present invention, I have found that the use of particular compositions comprising a random copolymer of ethylene and a vinyl trialkoxysilane and a flame retardant mixture, optionally, containing certain additives results in the formation of a flame retardant polymeric composition that when used as a primary insulating coating, enables the finished product to pass the vertical wire (VW-1) flame test and glancing impact test requirements designated by Underwriters Laboratories Inc (UL) for XHHW cable applications. The former requires that a vertical specimen of an insulated conductor shall not flame longer than 60 s following five 15-s application of flame, the period between applications being 15 s. The test specimens must not ignite combustible materials in the immediate vicinity or damage more than 25% of the indicator flag (UL 1581 1080.1-1080.9). The latter requires that the breakdown potential of each of six specimens of finished solid No. 14 AWG Type XHHW wire that have separately been subjected to a glancing impact (45.degree.) of 2 J or 0,207 m-kgf shall not be less than 20% of the average breakdown potential of six adjacent specimens of the same wire not subjected to impact (UL 1581, 700.1-700.6 ).